1. Field of the Invention
The present invention relates to optical wiring technology. More particularly, it relates to a method for manufacturing an optical coupling element for providing an optical coupling structure that interfaces between optical devices mounted on a substrate and optical waveguides formed on the substrate.
2. Description of Related Art
In recent years, high-end server systems have introduced a technology for interconnected processors and cores using optical wiring to achieve higher-speed information processing. As the performance of the central processing unit (CPU) of a server system increases, packaging density and the number of CPU cores increase, thus increasing the number of channels of an optical data link per unit processor-core. Under such circumstances, development of a technology for interconnecting devices, such as processors and memories, at high speed and high density is being required.
As a promising candidate of the technology for interconnecting devices at high speed and high density, as described above, a technology for achieving a data link among chips on a printed circuit board (PCB) through optical waveguides formed on the surface of the PCB attracts attention. However, this technology has a problem in achieving the interface between the optical waveguides formed on the board and chips on the board at high efficiency.
FIG. 14 shows a cross sectional view of an optical coupling structure for an optical waveguide and the outside according to related art. The optical coupling structure 500 shown in FIG. 14 includes an electrical wiring board 502 on which electrical wiring is provided and an optical wiring layer 506 formed on the surface of the electrical wiring board 502. The optical wiring layer 506 includes a core 508 that transmits an optical signal and clad layers 504a and 504b formed so as to enclose the core 508. The optical wiring layer 506 has, in the path of the core 508, an end face 512 perpendicular to the optical axis of the core 508 and to the top surface of the electrical wiring board 502 and a reflecting surface 510 having an inclination angle of 45° cut out. The reflecting surface 510 and the end face 512 are formed by laser beam machining, and the reflecting surface 510 is masked with gold, aluminum, or the like by vapor deposition.
With the optical coupling structure 500 shown in FIG. 14, light that propagates in the core 508 travels in the core 508 in the direction indicated by the dotted-chain line, exits from the end face 512, is incident on the reflecting surface 510 at an incidence angle of 45°, where it is reflected at a right angle to the outside of the board. The light exiting to the outside is introduced into a receiver or the like on the board. If a transmitter or the like is provided on the board, light exiting from the transmitter is incident on the reflecting surface 510 at an incidence angle of 45°, where it is reflected at a right angle into the core 508 through the end face 512 and propagates in the core 508 reversely. In this manner, with the optical coupling structure 500 shown in FIG. 14, optical devices mounted on the board, such as a receiver and a transmitter, and the optical waveguide of the board are interfaced.
Another method for forming a reflecting surface for an optical communication interface includes technology disclosed in Japanese Unexamined Patent Application Publication No. 2001-195771. A micromirror is formed on a silicon substrate by anisotropic etching or forming a half-mirror by forming an optical waveguide in contact with a micromirror surface formed on a silicon substrate by anisotropic etching and transferring the shape of the micromirror to an end face of the optical waveguide. Furthermore, Japanese Unexamined Patent Application Publication No. 2006-259590 discloses a technology for forming a 45° mirror surface by cutting a submount at an angle of 45° with respect to the optical axis using a dicing saw with an angle of 45°.
In the optical coupling structure 500 shown in FIG. 14, the reflecting surface 510 and the end face 512 are processed to about 50 μm to several millimeters in size. The reflecting surface 510 has been formed by laser beam machining. However, the laser beam machining degrades in machining performance for inclined surfaces and significantly degrades in machining performance, in particular, for minute regions. Therefore, the method is not enough to obtain a reflecting surface with high flatness, resulting in increasing a reflection loss.
The technologies disclosed in Japanese Unexamined Patent Application Publications No. 2001-195771 and No. 2006-259590 form a reflecting surface on a silicon substrate. Accordingly, those technologies neither form the reflecting surface in the optical wiring layer 506, as shown in FIG. 14, nor dispose a reflecting surface formed in a silicon substrate in the optical wiring layer 506. In addition, with the technology of the Japanese Unexamined Patent Application Publications described above, since the reflecting surface itself is formed by being cut with a dicing saw, it is not enough because it is difficult to obtain a reflecting surface with high flatness, thus resulting in an increase in reflection loss.
Thus, it has still been required to develop a technology for interfacing between optical waveguides formed on an optical transmission substrate and optical devices on the optical transmission substrate at high efficiency with a low reflection loss.